In Greek mythology, the Chimera was a fire-breathing monster with the head of a lion, the body of a goat, and the tail of a serpent. The Chimera was killed by Bellerophon mounted, on Pegasus, the winged flying horse. In medicine a chimera is a person with two or more genetically distinct lines of cells. Human chimeras were discovered with the advent of blood typing when it was found that some people had more than one blood type. Most of them proved to be blood chimeras – non-identical twins who shared a blood supply in the uterus. Those who were not twins are thought to have blood cells from a twin who died early in gestation. Twin embryos often share a blood supply in the placenta, allowing blood stem cells to pass from one to the other and take a new home in the bone marrow of the other twin. About 8% of non-identical twin pairs are chimeras. Many more people are microchimeras and carry small numbers of foreign blood cells that may have passed from the mother across the placenta, or persist from a blood transfusion. In vitro fertilization (IVF) is also contributing to the number of human chimeras as, to improve success rates, two or more embryos are placed in the uterus so that women who have IVF have more twin pregnancies than usual. More twins mean more chimeras.